Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field and subjected to a uniform magnetic field (polarizing field B0), the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). When a substance such as human tissue is the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency.
An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body.
Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. As the nuclear spins then relax back to their rest energy state (i.e., after the excitation signal B1 is terminated), they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
During an MRI scan, the MRI system experiences mechanical vibrations caused by various external and internal sources. For example, vibrations may be caused by the strong magnetic field environment and various elements of the MRI system, such as the coldhead motor or gradient coil (e.g., as a result of pulsing of the gradient coil), and by external sources such as floor vibrations caused by a nearby elevator or subway. The mechanical vibrations of such sources can cause the mechanical vibration of other elements inside the MRI system, such as the cryostat thermal shield, and induce eddy currents in electrically conductive material in the cryostat (e.g., the vacuum vessel, thermal shield, helium vessel). Such vibrations cause eddy currents to be induced on the metal structures of the MRI system. The eddy currents induce a magnetic field that is superimposed on the original homogeneous magnetic field generated by the MR system, which negatively affects the magnetic field homogeneity, causes artifacts on the image, and deteriorates image quality. The higher the main magnetic field is, the higher the induced eddy current will be and hence the higher the magnetic field distortion.
Attempts have been made to mitigate eddy current formation in MR systems by using software compensation models, providing vibration isolation pads, or designing the suspension system and components of the magnet structure to have a high stiffness to resist vibrations. However, such methods may not adequately mitigate induced eddy currents. Such methods also add cost and complexity to the MR system and increase the computational complexity of the image reconstruction process.
It would be desirable to provide a system and apparatus to passively (e.g., automatically) cancel or reduce the magnetic field distortion caused by eddy currents induced by mechanical vibrations.